EGR is commonly used in engine systems to help reduce engine gas emissions, such as nitrogen oxides (NOx). NOx is formed at high combustion temperatures and, when combined with hydrocarbons, leads to the production of smog. In an engine system employing EGR, a portion of exhaust gases are recirculated from the exhaust system to the intake system. Since exhaust gases have already combusted, they do not burn again when they are recirculated. As such, the inert exhaust gases displace some of the intake air to effectively slow and cool the combustion process and, thus, reduce NOx formation.
Precise control of recirculation flow may be desired since, for example, too much flow may retard engine performance, while too little flow may result in increased NOx production. Backpressure within the exhaust system, particularly in the vicinity of the EGR conduit, may impact the amount of flow through the EGR system, with a relatively high backpressure resulting in too much flow through the EGR system and a relatively low backpressure resulting in too little flow through the EGR system. Tuning engines to achieve desired recirculation flow may be further complicated by the fact that, due to manufacturing tolerances, even engines of the same model may experience different pressures and, thus, may exhibit varying recirculation flow.
U.S. Pat. No. 6,263,672 to Roby et al. discloses the use of an EGR valve and a restrictor valve in an engine system. The EGR valve provides a means for controlling the flow of recirculated exhaust gas to be provided to an intake passageway in response to an input from a controller. The restrictor valve is positioned upstream from a turbocharger turbine and restricts the flow of exhaust gas into the turbine. This restriction results in an increase in pressure of the exhaust gas provided to the restrictor valve. The higher pressure exhaust gas is provided to the inlet of the EGR valve, which may be actuable independently of the actuation of the restrictor valve. Although potentially effective, such an arrangement employs the use of electronically controlled valves, which may be costly and complex.
The present disclosure is directed to one or more of the problems set forth above.